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Real Events
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Predictions
Abstract Version
of Real Events
Model
Fig. 1.1
organization of our thoughts and data and in the evaluation of our knowledge about
the mechanisms that lead to the system's change.
Some people raise philosophic questions as to why one would want to model
a system. As pointed out earlier, we all perform mental models of every dynamic
system we face. We also learn that in many cases, those mental models are inade-
quate. With a formal model at hand—a model that is transparent enough for others
to understand and critique, and one that can be run over and over again to reveal its
behavior under different assumptions—we can specifically address the needs and
rewards of modeling.
Throughout this topic, we encounter a variety of nonlinear, time-lagged feedback
processes, some with random disturbances that give rise to complex system behav-
ior. Such processes can be found in a large range of systems. The variety of models
in the companion topics of this series naturally span only a small range—but the
insights on which these models are based can (and should) be used to inform the
development of models for systems that we do not cover here.
It is our intention to show you how to model, not how to use models, nor how to
set up a model for someone else's use. The latter two are certainly worthwhile ac-
tivities, but we believe that the first step is learning the modeling process. In the fol-
lowing section, we introduce you to the computer language that is used throughout
the topic. This computer language will be immensely helpful as you develop an un-
derstanding of dynamic systems and use that understanding to solve new problems.
1.2 Static, Comparative Static, and Dynamic Models
Most models fit in one of three general classes. The first type consists of models that
represent a particular phenomenon at a point of time—these are static models. For
example, a map of the United States may depict the location and size of a city or
the rate of infection with a particular disease, each in a given year. The second type
is the set of comparative static models that compare some phenomena at different
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